The present invention relates to a method for improving the hardness and strength of natural products such as pearls, corals, shells, etc. which are composed mainly of calcium carbonate, and also to a method for providing them with resistance against acids, alkalis and salts, which increases the value of decorative accessories, such a necklace, ring, pendant, etc. made of such natural products.
Pearls, corals and shells have been widely used as accessories, such as a necklace, ring, pendant, etc., because they have a natural luster and they are rare. However, those natural products have the drawbacks that they are weak mechanically and have insufficient resistance against salt because the major component of those natural products is calcium carbonate. Accordingly, they are apt to be readily broken when they are dropped, they tend to crack when contacted with other hard objects, and they are readily damaged by salt in perspiration. Besides, there is the unfavorable possibility that the original luster characteristic of the natural products will be spoiled if a protective film is formed on the surfaces of those natural products in order to improve their hardness and to provide resistance against acids, alkalis and salts. A method for reinforcing and improving those natural products has been sought.